Xinyu Luo

Deterministic entanglement generation from driving through quantum phase transitions

Transitional approach to entanglement In an entangled many-particle system, changing the state of one constituent affects the rest of the system. This property can be used as a resource in quantum information processing, but getting many particles to participate in entanglement is tricky. Luo et al. used another collective phenomenon, a quantum phase transition, to entangle more than 900 atoms in a Bose-Einstein condensate. The size of the entangled ensemble remained stable, making the approach practical for precision measurements. Science, this issue p. 620 A large entangled 87Rb atom ensemble robust to number fluctuations was created. Many-body entanglement is often created through the system evolution, aided by nonlinear interactions between the constituting particles. These very dynamics, however, can also lead to fluctuations and degradation of the entanglement if the interactions cannot be controlled. Here, we demonstrate near-deterministic generation of an entangled twin-Fock condensate of ~11,000 atoms by driving a rubidium-87 Bose-Einstein condensate undergoing spin mixing through two consecutive quantum phase transitions (QPTs). We directly observe number squeezing of 10.7 ± 0.6 decibels and normalized collective spin length of 0.99 ± 0.01. Together, these observations allow us to infer an entanglement-enhanced phase sensitivity of ~6 decibels beyond the standard quantum limit and an entanglement breadth of ~910 atoms. Our work highlights the power of generating large-scale useful entanglement by taking advantage of the different entanglement landscapes separated by QPTs.

Tunable atomic spin-orbit coupling synthesized with a modulating gradient magnetic field.

We report the observation of synthesized spin-orbit coupling (SOC) for ultracold spin-1 87Rb atoms. Different from earlier experiments where a one dimensional (1D) atomic SOC of pseudo-spin-1/2 is synthesized with Raman laser fields, the scheme we demonstrate employs a gradient magnetic field (GMF) and ground-state atoms, thus is immune to atomic spontaneous emission. The strength of SOC we realize can be tuned by changing the modulation amplitude of the GMF, and the effect of the SOC is confirmed through the studies of: 1) the collective dipole oscillation of an atomic condensate in a harmonic trap after the synthesized SOC is abruptly turned on; and 2) the minimum energy state at a finite adiabatically adjusted momentum when SOC strength is slowly ramped up. The condensate coherence is found to remain very good after driven by modulating GMFs. Our scheme presents an alternative means for studying interacting many-body systems with synthesized SOC.

Ultra-High Efficiency Magnetic Transport of 87Rb Atoms in a Single Chamber Bose?Einstein Condensation Apparatus

We report the ultra-high efficiency transport of cold 87Rb atoms using a moving magnetic quadrupole potential generated by three overlapping pairs of fixed coils. The transfer efficiency is better than 97%, which is the highest ever reported to our knowledge. The temperature increase due to heating is less than 10 when the initial cloud temperature is 110 μK. Our setup is similar to the magnetic transferring belt design [Phys. Rev. A 63 (2001) 031401(R)], although it is simpler because the push coil is not required. We use it to transport atoms away from a magneto-optical trap to very close to the wall of the glass cell, facilitating future experiments employing three-dimensional optical lattices, high resolution in-situ imaging, and magnetic Feshbach resonances.

Beating the classical precision limit with spin-1 Dicke states of more than 10,000 atoms

Significance Entanglement is central to studies in foundations of quantum mechanics, quantum information, and precision measurement. Among the variety of multipartite entangled states, Dicke states form an important class, and their realizations attract widespread interest. Most of the Dicke states produced to date are limited to pseudospin-1/2 (two-level) particles. This work reports the generation of balanced Dicke states comprising spin-1 (three-level) atoms and the subsequent demonstration of enhanced interferometric sensitivity over the standard quantum limit facilitated by them. We expect it will stimulate both experimental and theoretical research efforts on entangled states of higher-spin particles. Interferometry is a paradigm for most precision measurements. Using N uncorrelated particles, the achievable precision for a two-mode (two-path) interferometer is bounded by the standard quantum limit (SQL), 1/N, due to the discrete (quanta) nature of individual measurements. Despite being a challenging benchmark, the two-mode SQL has been approached in a number of systems, including the Laser Interferometer Gravitational-Wave Observatory and today’s best atomic clocks. For multimode interferometry, the SQL becomes 1/[(M−1)N] using M modes. Higher precision can also be achieved using entangled particles such that quantum noises from individual particles cancel out. In this work, we demonstrate an interferometric precision of 2.42−1.29+1.76 dB beyond the three-mode SQL, using balanced spin-1 (three-mode) Dicke states containing thousands of entangled atoms. The input quantum states are deterministically generated by controlled quantum phase transition and exhibit close to ideal quality. Our work shines light on the pursuit of quantum metrology beyond SQL.

Generating an effective magnetic lattice for ultracold atoms

We present a general scheme for synthesizing a spatially periodic magnetic field, or a magnetic lattice (ML), for ultracold atoms using pulsed gradient magnetic field (GMF). Our scheme is immune to atomic spontaneous emission often encountered in optical lattices, and has the additional benefits of easy tunability for both the lattice period and depth. Technical requirements for the experimental protocol implementing our scheme is estimated and shown to be readily available in today’s cold atom laboratories. The effective Hamiltonian for atoms interacting with the synthesized two-dimensional ML has not been studied in quantum condensed matter physics previously. Its band structure shows interesting features reminiscent of lattice models in p-orbit physics. Realization of our proposal will significantly expand the repertoire for quantum simulation with ultracold atoms.

Harmonic trap resonance enhanced synthetic atomic spin-orbit coupling

Spin-orbit coupling (SOC) plays an essential role in many exotic and interesting phenomena in condensed matter physics. In neutral-atom-based quantum simulations, synthetic SOC constitutes a key enabling element. The strength of SOC realized so far is limited by various reasons or constraints. This work reports tunable SOC synthesized with a gradient magnetic field (GMF) for atoms in a harmonic trap. Nearly ten-fold enhancement is observed when the GMF is modulated near the harmonic-trap resonance in comparison with the free-space situation. A theory is developed that well explains the experimental results. Our work offers a clear physical insight into and analytical understanding of how to tune the strength of atomic SOC synthesized with GMF using harmonic trap resonance.

Atomic spin orbit coupling synthesized with gradient magnetic fields

Spin orbit coupling (SOC) for neutral atoms can be synthesized with pulsed or time modulating gradient magnetic field (GMF). This is confirmed through the studies of collective dipole oscillations for a spin-1 atomic condensate in a harmonic trap after abruptly turning on SOC and adiabatically adjusted equilibrium states when SOC strength is slowly ramped up. Further measurements reveal that SOC can be enhanced when the GMF modulation frequency approaches harmonic trap frequency. Additionally, we discuss how the technique of pulsed GMF can be used to synthesize space-period magnetic fields, or magnetic lattices.

Studies of atomic spin-orbit coupling synthesized with gradient magnetic fields

Atomic spin orbit coupling (SOC) can be synthesized with modulating gradient magnetic field (GMF). This is confirmed through studying collective dipole oscillations for a spin-1 atomic condensate in a harmonic trap after abruptly turning on SOC, and studying adiabatically adjusted equilibrium states when SOC strength is slowly ramped up. Further measurements reveal that SOC can be enhanced when the GMF modulation frequency approaches trapping frequency.